Antenna excitation system



Sept. 30, 1952 R. s. W EHNER ANTENNA EXCITATION SYSTEM 4 Sheets-Sheet 1 Filed Oct. 14, 1947 FIG. 3. FIG. 4.

I F Us. 2.

INVENTOR. ROBERT S. WEHNER BY 6 Z I I M A?TORNEYS.

Sept. 30, 1952 R. s. WEHNER ANTENNA EXCITATIONN SYSTEM 4 Sheets-Sheet 5 Filed Oct. 14, 1947 5 a M N m m w a a E E M n R Z 7 a u w p x M u 5 M d a Z a r P b PIT P L l I h muzvkbcmmx WQSQUYMQ .WQERNUQQQ 7 8 9 G s. G F F F HO MC Llllrll r A TORNEYS.

Sept. 30, 1952 R. s. WEHNER ANTENNA EXC-ITA'TION SYSTEM Fil ed Oct. 14, 1947 4 Shgets-Sibg et 4 FIG. l4.

ROBERT '5v WEHNER A ORN EYS.

Patented Sept. 30, 1952 Robert Stephen wan signer to Airborne Inc., Mineola, N. Y.

er, Hempstead, N. Y., as- Instruments Laboratory,

Application October 14, 1947, Serial No.'779,816

(o1. est--33)v 10 Claims. 1 i This invention relates to antenna systems for .the radiation and reception of radio waves. It relates more particularlyto aircraft antennas and to methods and apparatus for feeding radio energy to such antennas. 1 e 1 The trendof development. of modern aircraft has been toward increasingly-higher flight speeds and, accordingly, toward more exacting airfoil design. At the same time the amountand complexity of electronic equipment carried in.-.'the plane for communication, navigation, collision warning, instrument landing etc.,-'hasincrea'sed. In operation, much of this equipment requires the radiation or reception of radio waves.and, accordingly, requires suitable *ante'nna structures capable of efficient performance over the'wide range of "frequencies utilized by the electronic equipment. The increased flight speeds of the aircraft and the resulting necessity for careful aerodynamic design render any externally projecting antenna structures that would alter the configurations of the aircraft surfaces. 1

It has become, therefore, ofparamount'importance to provide aircraft antenna systems which require no externally projecting or'protruding portions. Many such faired-in or recessed antennas have been developed. These antennas, although operating satisfactorily" for many applications in the superhigh frequency ranges, e. g. above 500 megacycles, generally-have been unsuitable for operation, in the lower frequency ranges. One reason for this unsatisfactory operation is that the. radiating structures are necessarily'larger for these lower frequency, 1. e. longer Wavelength, antennasthan. are those required for the higher frequency ranges, and it is, accordingly, more diflicult to provide sufficiently large openings in the surface of the aircraft to permit the use of conventional cavity or slot type antennas.

Because of the large number of radio equipments which must be. operated in the plane-and because of the large radiating structures required at these lower frequencies it is desirable thatithe -faired-in antenna operate over a-wide range of frequencies so that various electrical equipments may be connected as desired to the same'antenna structure; andfor many applications it would be desirable that no readjustment of I antenna matching networks be required so that bythe use of suitable band-pass filters several equipments ,could be connected 1 simultaneously, with theantenna.

,As the frequency of operationis reducedT the= undesirablethe use of I fiat surfaces of the aircraft suitable for ground surfaces, i. e. electrically conductive surfaces adjacentthe antenna, become increasingly smaller electrically, and it is, therefore, essential that the antenna be constructed and driven (i. e. electrically energized) so that satisfactory operation may be achieved inconjunction with a relatively smallground plane,;and that the effects of this ground plane be minimized so that the antenna impedance presented to'the feedline is not altered excessively by frequency changes.

within the operating range.

The impedance presentedto: the antennafeedline should be suii'iciently'highthat; ordinary coaxialline maybe used to couple energy between the transmitting or receiving equipment andthe antenna. If it is found necessary toempl'oy impedance matching elements .in-conjunction with the antenna, it generally is'desirablethat such elements operatexwithout reaojustmentover the entire frequency range with whichtheantenna is to be used. Such readjustment isinconvenient and precludessimultaneous. use;'of the antenna with more than one .electrical equipmenta *In accordance with-thapreferred; embodiment ofv the present invention a completelyfaired-in antenna is provided that is2operab1e lover a wide frequency range and.,-which makes -use' -of.; al-' .ready existing aerodynamic;,. surfaces asthe radiating elements. This. antenna provides vertically polarized, radio: Waves, advantageous,

this frequency range, and is canablemfsat siaw tory operationin conjunction with nelectrcally} small ground surface .y;:

in elation-to It is, accordingly, an objec provide an improved faired-.in;aircraft antenna system. I

It is a furtherobjecta ofj this invention 1 vide such an antenna system inawhi ch the effects of the ground plane configuration on the .-.im.

pedance characteristicsof the antenna are minii e H It is anotherobject provide such 4 as system utilizing 'a' portionofthfe aiicraftstruc I I tically, 1 .1

ture'" as a radiating element to"provide yer In aspect e invenuon-iti iaa been I gsysto: provide 'an improved wide b temforaircraft antennas.' I a In another aspect ofthe invention it is ject'to provide an improved-broadband i/"e'rticalL- -ly polarized antenna system-in whi'ch' the driving impedance. is maintained".within -'praoticalvop' eratingxlimits of the characteristic impedanceof a conventionalztype coaxial feed Tl-ine over a wide I able {for receiving. I

with relation-toth'e radiation 'aind -reception of 3 3 "range of frequencies, or can be brought within these limits by an easily realizable non-adjustable impedance matching section.

The various features of novelty which characterize the present invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understandingof the invention,its advantages and the specific objects obtained with its use, reference should be had to the following descriptive matter and to the accompanying drawings, inwhich:

Fig. 1 shows the tail structure of an airplane with the position of the antenna-excitation element indicated in broken "lines;

Fig. 2 is an enlarged cross-sectional view of the tail fin and excitation element;'- 7

Fig. 3 is an enlarged cross-sectional view of the excitation element taken on line 3-3 of Fig; 2';

Fig. 4 is an enlarged cross-sectional view taken on line 4--4 'of'Fig. 2; 1 r

Fig. 5 shows the excitation element and the upper portion of the tail'fin with the plastic-tail cap removed;

Fig. 6 shows the upper portion of the tail :fin

with the plastic 'tail cap in place;

Figs.7, 8 and 9 show'the input impedance of one particular antenna structure and the change produced therein by simple matching sections;

Fig. 10 shows the standingwave ratio on the feedlin'e 'of atypical antennastructure embodying the invention;

Figs. 11, 12 and13 show'typic'al' field strength patterns of the'radiating'structure for the indicated ra'diation angles near the horizontal'plane; and,

, Fig. 14 shows a folded-loop excitation element for operation over a narrower'frequency range.

The characteristics of a given antenna 6 are the same, if properly interpreted, regardless of whether that antenna 'is' used for transmitting orreceiving. This equivalence follo'ws'the reciprocity law which is an extension 'of the wellknown reciprocity theorem" found in. "electrical circuits; Although the discussion herein I refers for 'the 'most part'to transmitting antennas, the

interpretation of the present disclosure is not to be so limited; The'term radiating element, therefore, is to be 'co'nstrued-notas an element necessarily used for radiating, but rather one whichis capable of radiating and,-therefore, suit-" Other terms used herein,

electro-magnetic energy; are to be similarly-interprete'd. I

In order to provide afaired-in radiating felement capable of satisfactory operation in the frequency range between- 50 and 400 mega'cy'cles, a

, metal vertical stabilizer} (Fig. 1} r tau fin, of the aircraftmayi be utilizedadvathtagebusly as the radiating elemen't: "One 'difiic ulty is theft 3 theground surfaces formed by the horizontal fstabiliz'ers 4 math -ad acentperms-n of the fuseiage' arelshia'll'ih terins Of the'waivlflgths 1 at which the dimensions of the tail fin o t s ab e a a reds- In order to minimize theefiects of theseground surfaces, the tail fin 2 is excited or fednear the upper end. .Withsuch topfeedingthe'impedance presented to the. feed line, which is determined qlargely by; the surfacecurrentsimmediatelyad 1 jacent'thepolnt'of feedydo'es n'ot'change as rapidly with changes in frequency as if I it were fed 7 near the base where the small and discontinuous make it This structure is closed at the top by a metal plate l2 (Fig. 5) suitably secured to the upwardly extending-sides of stabilizer 2. An excitation element 14, mounted above the stabilizer 2, is sup- I ported from plate [2 by insulating supports l6 and I8 constructed, for example, of impregnated Fiberglas.

To avoid changing the shape of the airfoil the excitationelement l4 isshaped to-conform generally with the desired surface configuration and .covered with a housing 22 (Fig. 6) of plastic or other suitable insulating material, shaped to provide the desired aerodynamic characteristics. To transfer energy from the transmitting or receiving equipment (not shown) to the excitation element It, acoaxial'cable 24 (Fig. 2) extends from this equipment upwardly through the interior of the vertical stabilizer 2 to the excitation element 14. The outer conductor of the co- .axial cable 24 is grounded to the plate I2 and the inner conductor is'connected to the excitation element.

.The radio frequency energy delivered by the coaxialcable 24 to the excitation element I4 is coupled by capacity to the radiating surfaces of the vertical stabilizer 2. In order to reduce'the direct, or shunt, capacity which is efiectively between the inner conductor of cable 24 and the metal'su'rfaces of stabilizer 2, a tapered section, generally indicated at 26, forms the lower part of element I 4. The tip of this tapered section 26 joins the inner conductor of cable 24 near the surface of plate l2 and with gradually increasing dimensions upwardly, forms the excitation element.

. The shape of element 14 is shown by Figs.'3, 4, and 5. The-exact shape is not critical and again is governed to considerable extent by aerodynamic designconsiderations. It'must, however, be sufficiently large to provide adequate series capacitive coupling- .between cable 24 and the-surfaces 'of the vertical stabilizer 2, but in orderthat satisfactory operation may be had over a'wide frequency range the dimensions of excitation-element- 14 are small in terms of wavelengths at the highest frequency of operation.

, rudder portion.

The antennas described above can be readily matched to a coaxial line having a' ohmcha'n- 'acteristic impedance so that there is less than a two to one voltage standingwave ratio on the line over wide frequency ranges, provided, the-height and-cross sectional! dimensions" of the 'fin' are sufficiently large as compared "to a wavelength at "the minimum frequency of :operatiom andthat the ratio of the height of the excitation element to: theiover all height in wavelengths is such as tially throughout this frequency thereto by an easily realizable transforming section.

' It has been observed that the vertical height of the stabilizer 2, the height of the excitation element M, the length of the tapered portion 26, and the over-all height of the structure, are the more critical factors in determining the antenna impedance, whereas reasonable changes in the shape and other'dimensions of the radiating 'element and in the excitation element I4 produce impedance changes of secondary importance.

Unexpectedly, the input impedance of this antenna is similar to that of a sleeve antenna with a comparable ratio of sleeve to over-all height and operating in conjunction with a large flat ground plane. This is particularly surprising because the combined area of the horizontal stabilizers and the upper adjacent surfaces of the fuselage are small in termsof square wavelengths and are far from continuous surfaces. 7

The useful operating range over which the antenna may be made to operate satisfactorily depends upon the height of the vertical stabilizer.

Best operation is obtained readily when the height of the vertical stabilizer is less than 0.4 or more than 0.6 wavelength. Advantageously, for wide band operation, the height of the stabilizer should be, therefore, at least 0.6 wavelength at the minimum frequency of operation, or itshould be less than 0.4.- wavelength at the maximum frequency of operation.

However, satisfactory but relatively narrow band antennas of this type may be constructed .for operation in this intermediate range, i. e.

where the stabilizer height is between 0.4 and 0.6 wavelength.

One particular antenna, constructed in accordance with the present invention and designed to operate over the'frequency range from 100 to 250 megacycles, presents an input impedance over this frequency range as shown by line 32 in Fig. '7. As shown, the standing wave ratio on a 50 ohm coaxial transmission line was greater than two to one over most of the frequency range. However, the addition of a simple series condenser having a value of 19 pmf. transforms the input of this antenna to that shown in Fig. 8. This impedance curve is further transformed tothat shown in Fig. 9 by the addition of a 100 ohm series line section having a length of approximately 22 degrees at 100 megacycles. Thus, as shown in Figs. 9 and 10, the standing wave ratio on the 50 ohm line is less thantwo to one substanrange.

Field patterns of this antenna at the mid-frequency of operation are shown in Figs. 11, 12 and 13. Fig. 11 shows the field pattern of the antenna 20 degrees below the horizontal plane; Fig. 12 indicates the pattern degrees below the horizontal; and Fig. 13 shows the field pattern at an angle of 10 degrees above the horizontal plane.

It is apparent that the antenna may be constructed by insulating the top portion of the vertical stabilizer. This may be done by cutting the metal stabilizer along a horizontal line and then connecting the surfaces and internal struts of the two stabilizer sections with insulating ma-- terial having the requisite strength. Thus thetop portion of the vertical stabilizer is separated by short lengths of insulating material from. the lower portion. The top section is fed in thesame manner as the excitation element l4 above.

Another type of excitation element is shown in Fig. 14, and has particular utility for appli cations in which the height of the plastic housing 22 is limited and where extremely wide band operation is not required. In this example, an excitation element is formed by a strip of suitable conductive material, for example, inch diameter copper tubing having an over-all length substantially equal to one-half wavelength at the mid-frequency of operation. The inner conductor of the coaxial transmission line 24 is connected to one endof the excitation element 30 by a short tapered section 38; the opposite end of the copper tubing 36 is connected to the plate [2, as at E2, by screws or other suitable means.

In order to avoid adverse impedance changes? citation element 36 forms a folded loop, as shown in Fig. 14. With this arrangement, that portion of the excitation element 36 nearest the rudder is a relatively low current area and, thus, minimizes impedance changes caused by coupling and hence dependent upon the angular position of the rudder.

In this specification and the accompanying drawings, there is shown and described a preferred embodiment and one modification of the invention; but it is to be understood that these are not intended to be exhaustive nor limiting of the invention, buton the contrary are given for purposes of illustration in order that others skilled in the art may fully understand the invention and the principles thereof and the manner of applying it in practical use so that they may modify and adapt it in various forms, each as may be best suited to the conditions of a particular use.

I claim:

l. A broadband vertically polarized aircraft antenna system, comprising a vertical tail fin having an outer conductive surface portion extending upwardly from the fuselage of said aircraft, an excitation element mounted above and supported by said conductive portion, a coaxial transmission line having an inner and an outer conductor and extending from the interior of said aircraft through said conductive portion to the upper portion thereof, a tapered section forming a part of said excitation element and electrically connected to the inner conductor of said coaxial line.

2. A vertically polarized top-fed antenna system, comprising a source of electrical energy, a

ground plane, a radiator extending upwardly from said ground plane, a feedline connected to said source and extending'to the upper end of said radiator, and an excitation element mounted above and feeding electromagnetic energy to said radiator one end of said element being connected to said feedline and the opposite end to the upper portionof said radiator and having a total length therebetween equal to an integral multiple of half wavelengths substantially at the mid-frequency of the operating range.

3. A vertically polarized top-fed antenna system,.comprisi s.a round plane. aradiat rextendingupwardly rom s d r und, l n n netic energy to the structural portions of an aircraft to produceelectromagnetic radiation therefrom, comprising a vertical tail fin'of said aircraft having conductive and non-conductive portions, a source of electrical energy, a. feedline extending between the base and, the upper portionof said conductive portion, an excitation element mounted within said non-conductive portion-and coupling electromagnetic energy between said feedline and said conductive portion, said excitation element comprising a conductor having a total length substantially equal to onehalf Wavelength at the frequency of operation, one end of said excitation element being connected to said feedline and the opposite end being connected to the upper surface of said conductive portion- 5. A vertically polarized top-fed antenna system, comprising a ground plane, aradiator extending upwardly from said ground plane, an excitation el'ement mounted above and feeding electromagnetic energy to said radiator, said excitation element comprising a generally U- shaped length of conductive material, the midpointthereof being displaced inthe same horizontal direction from each end thereof and having an overall length substantially equal to onehalf wavelength at the mid-frequency of the operating range, and a feedline connected to one end of said excitation element, the opposite end of said element being connected to the upper surface of said radiator.

6. A coupling. system for feeding electromagnetic energy to the metal surface of. the tail structureof an aircraft having a laterally'movable vertical rudder, comprising a S0111IGe'Qfe1ectrical energy, a stationary vertical tail structure having conductive and non-conductive surface portions, a feedline extending between said source of energy and the upper end of said conductive portion, and, an excitation element mounted above and. coupling electromagnetic energy between said feedline and said conductive portion, said excitation element comprising a length of conductive material having a total length substantially equal to. one-half Wavelength at the frequency of operation, one end thereof being connected to said feedline and the opposite end thereof being connected to the conductive portion of said tail structure, said element having areas of high and other areas of lowcurrent density therealong and havingan area of low cur:- rent densitynearer. said movable, rudder than any area of maximum current density.

7. An antenna coupling system for usein the transmission and reception of radio Wavesin an aircraft of the type having an electrically conductive outer skin, compris ing a verticaltailfin connected to. and stationary with respect to the fuselage ,of said aircraft andhaving afirst elec? trically conductive airfoil portion and a second non condu'ctive airfoilportion, a feedline extend ing through said conductive portion, and an excitation. element mounted in a fixed position on the uppersurface of ,said conductive portion and confined within said non-conductive portion and connected to said feedline and coupling electromagnetic energy between. said feedline and said conductive portion.

8. An antenna coupling system for feeding electromagnetic-energy to an electrically conductive vertical structural element of an aircraft for radiation therefrom into surrounding space, -said system comprising a vertical structural element.

forming .an integral part :of the surface of said aircraft-and having a lower electrically. conductiveradiating surface portion and/an upper nonconductive surface portion, an excitation v e lennent mounted within said noneconductive portion. and

coupling ele ctromagneticenergy to said conductive portion, a tapered portion connected ,to. said excitation element, and.'a' ,feedline connected to said tapered portion of said'excitation-element and extending throughsaid conductive portion. 9. An aircraft antenn'a systemcomprising .a vertical airfoil having' an exposed electrically conductive portion and an exposed insulating portion, the outer end .offsaid conductiveportion being closed by and filectrically continuous. with a conductive plate cal bed by said insulating portion, a feedline Manama and second. conductors and'extendingffrom the interior offsaid aircraft. to. said plate, said-first conductor being connected to. saidplate, and an excitation element mounted within said insulating. portion and comprising a loop Ioffcon'duc'tive material extending between said 's'econdconduetor andjsaid plate; v 10. A broadband vertically polarizedaircraft antenna, system, comprising. .a' vertical stabilizer extending. outward1y .from the fuselage of said aircraft and havingfa'ri outer exposed conductive surface portion electrically continuous 'with the outer surface of saidgaircraft.fuselage,an excitation element. supportedby. the outermost portion of said conduotiveiportion and extending beyond said conductive portion a distance equal to at least 0.10 of the total height of, saidstabili'zer, impedance matching, means, connected to fsaid excitation element and positioned'foutsi'de said conductive portion; a .fee dline extending between Said mpeda mat h ng m s and the interior of said aircraft, and a' streamlined. non-conductive housingsurrounding at least part of said excitation elementandlsupporl ed by said conductive portion,

" RoBEarsTEP N REFERENCES-CITED: The following references are of record in the file of this patents a v UNI TE I? STATES PATENTS Number 1 v w w l I Name Date 2,210,066v Co rk.. Aug...6, 1940 2,235, 39; .Bruce Mar. 18, 1941 2,242,200 Woods 'May 13, 1941 13,046 Bruce Mar. 9, 19.43 2,334,279 Neiman 1 Nov. 16, 1943 2,344,171 I Rote ..Ma1t. l4, 1 944 2,417,793 Wehner,1 Mar.18, 194.7 2,468,547 Meier Marat, 19.49 

